We propose to determine involvement of thyroxine in the earliest steps of gene induction during differentiation and development of Rana catesbeiana. Radioactive thyroxine will be bound to cytosol and nuclear fractions prepared from tadpole tissues. The research program has a three-part design: to determine the subcellular sites of action for thyroxine at the organelle and molecular levels, to describe the nature of the interaction between hormone and macromolecules, and to propose a model to account for multiple developmental effects induced by the same hormone in one animal. The minimal concentration necessary for developmental changes in the liver and tail will be determined from thyroidectomized tadpoles. Then the receptor will be distinguished from the storage and metabolic sites for the hormone by an analysis of the subcellular distribution of radioactive thyroxine as a function of the hormone concentration. The binding of low concentrations of radioactive hormone with and without competing unlabelled hormone analogous to soluble cytosplasmic proteins and nuclear components will allow the identification of possible hormone receptors. Preliminary experiments have demonstrated the existence of one such high-affinity binding protein in the cytosol as a candidate for the receptor. Since the accumulation of unmetabolized thyroxine by nuclear chromatin appears to be a necessary step of development, we propose to study the possible involvement of cytosol binding proteins in the accumulation of thyroxine by the cell nucleus. Binding in tadpole liver and tail tissue will be compared to determine if a single conceptual model can account for the initiation of cell maturation and cell regression.